Formation of Metallic Mercury During Photodegradation/ Photodarkening of α-HgS: Electrochemical Evidence

摘要

Since antiquity, red mercury sulfide (α-HgS), called cinnabar in its natural form and vermilion in its synthetic form, has commonly been used as a pigment. An undesirable phenomenon is the degradation of this bright-red material in the presence of light, chloride ions, and humidity, causing it to turn black. This degradation phenomenon has been observed on the surface of frescoes at important heritage sites such as Pompeii, and of paintings from famous masters such as P. P. Rubens and P. Brueghel. An appropriate conservation of this valuable and irreplaceable heritage requires a profound knowledge on the degradation mechanism of the pigment. Several methods, such as X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and secondary-ion mass spectrometry (SIMS) have been employed to identify the degradation products: HgCl2, Hg2Cl2, HgSO4, Hg2SO4, and Hg3S2Cl2. Yet, none of these compounds has a dark color that can explain the blackening of α-HgS in a convincing manner. Several hypotheses for the decomposition and discoloration have been proposed, some assuming the formation of black β-HgS and others that of metallic mercury. While volatile mercury has already been observed in photodegradation experiments on mercury orc, neither β-HgS or metallic mercury as a deposit have been detected on naturally and artificially degraded HgS paint. Moreover, the role of chloride ions is not fully understood. This study presents the results of electrochemical experiments that demonstrate for the first time the formation of metallic mercury as a degradation product of HgS induced by the joint action of light and chloride ions. A degradation mechanism consistent with these findings is proposed.